The present invention relates to an apparatus for storing a multi-component cryogenic liquid within a storage tank. More particularly, the present invention relates to such an apparatus in which headspace vapor within the storage tank is condensed by indirect heat transfer with the cryogenic liquid. More particularly, the present invention relates to such an apparatus in which the headspace vapor is condensed within an external condensation tank and the resulting condensate is remmed to the storage vessel by a pressure building circuit.
Cryogenic storage vessels and dewars are used to store cryogenic liquids, for instance, liquefied atmospheric gases, either at their point of use or for use in the transport of such cryogenic liquids. Although such storage tanks and dewars are insulated, there is still heat leakage into the storage tank or dewar. This heat leakage causes vaporization of the liquid cryogen. Typically, the vapor is vented from a headspace region of the tank to prevent overpressurization of the tank. Where the liquid cryogen is a multi-component mixture, for instance air, the venting of the vapor phase presents a problem because the more volatile components will vaporize before the less volatile components. As a result, the liquid being stored will have an ever increasing concentration of the less volatile components. For instance, if the liquid cryogen being stored is liquid air, nitrogen (as well as other components of the air but at a lower concentration) will be vented to cause the liquid to have an ever increasing oxygen content.
In order to overcome this problem, U.S. Pat. No. 3,260,060 discloses a cryogenic dewar in which liquid is vented through a heat exchanger located within the headspace region of the dewar. As pressure within the dewar increases, the liquid passing through the heat exchanger condenses the vapor to stabilize the concentration of the liquid. Since the liquid, now vaporized, is at the same concentration of the bulk liquid, there is no concentration change.
The problem with the cryogenic dewar illustrated in U.S. Pat. No. 3,260,060 is that it involves manufacturing dewars with heat exchangers in the headspace region and thus, cannot easily serve as a retrofit to existing cryogenic dewars. As will be discussed, the present invention solves the retrofitting problem by providing a cryogenic storage apparatus that is easily adapted as a retrofit for conventional cryogenic storage tanks and dewars.